Merge in forked repo shows up in source repo

Someone forked my project and updated it, assumedly using a merge.
A “merge from master” notification showed up in my related issue.
With most forks, that does not happen, however, and I certainly do not want lots of actions in forked repositories to show up in my own issues, so 2 questions:
What’s the difference that triggers this behaviour?
How can I prevent it in my original repository?

Hi @mintty , welcome to the Support Community!

A user with write access to your repository can merge directly to the upstream (your parent repository).

If the user does not have write access, then they would have to create a pull request to be reviewed by the upstream repository owner.